The process of electrical discharge machining (EDM) is well known: it is a method to cut conductive materials with an electrode wire that follows a programmed path.
An electrode for EDM has to meet a number of requirements. In the first place, a good machinability, i.e. a high removal rate, a good surface finish and a high precision, is required.
Furthermore, in order to obtain an automated system, a good threadability is desired.
The machining conditions and the path along which the workpiece is cut, are stored in a control device memory.
During machining, due to the electric discharge, a force with an orientation opposite to the direction in which the machining is proceeding is created on the machining sections of the wire electrode.
As a consequence, the wire lags in the direction in which the machining is proceeding.
Also electrostatic and electromagnetic forces are created on the wire electrode.
Due to all these forces and due to the vibrations of the wire, the actual position of the wire is different from the programmed position. This results in accuracy and precision problems.
This is especially critical for corner cutting. Deviation from the programmed outline at the corners has as result that round corners are obtained in stead of the desired sharp corners.
The deviation between the programmed and the effective outline can be reduced by applying a mechanical load to the wire.
If a high degree of accuracy and precision is desired, the use of wires with a small diameter is desired.
However, since only a limited mechanical load can be applied on the conventional finer wires, such as brass wires, it is difficult to obtain a high degree of accuracy and precision.
Therefore, for example plain Mo or W-wires are used for high precision cutting. These wires have a rather high tensile strength (>1900 MPa) but they have the drawback to be expensive.
Furthermore, since they are characterised by a high melting temperature, they have a low vapor pressure with poor flushability.